\documentclass[tikz,convert=pdf2svg]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=0.8,thick]
\def\iangle{120}
\begin{scope}[xshift=-2cm]
\draw[->] (-1.2,0) --(1.2,0);
\draw[->] (0,-1.2) --(0,1.2);
\draw[thick] (0,0) circle(1cm);
\coordinate [label = \iangle:$P$] (P) at (\iangle:1);
\coordinate [label = below:$P0$] (P0) at (P |-0,0);
\draw (P)--(P0);
\draw (0,0)--(P);
\fill [fill = gray,fill opacity=0.2] (0,0)--(0:1) arc (0:\iangle:1)--cycle;
\filldraw [fill = gray,fill opacity=0.5] (0,0)--(0:0.3) arc (0:\iangle:0.3)--cycle;
 
\end{scope}

\end{tikzpicture}

\end{document}
